count(*)和count(columnName)之间的差异

Question

我想知道使用之间有什么区别:

SELECT email, COUNT( email ) AS total
FROM `newsletter`
GROUP BY email having total>1

要么

SELECT count(*) as total, email 
FROM 'newsletter' 
GROUP BY email having total > 1

两者都给出了相同的结果,但还有什么count(*)比电子邮件更重要？

Answer 1

至少有一个区别.

• 他们可能会返回不同的结果,如果email可以包含NULL.

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